Field of the Invention
Embodiments of the present invention relate generally to a pattern inspection apparatus and a pattern inspection method. More specifically, embodiments of the present invention relate, for example, to an inspection apparatus and method which inspects a pattern by acquiring a secondary electron image of a pattern image emitted because of irradiation of an electron beam.
Description of Related Art
In recent years, with the advance of high integration and large capacity of large-scale integration (LSI) circuits, the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by circuit formation of exposing and transferring a pattern onto a wafer by means of a reduced projection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask for transfer printing such a fine circuit pattern onto a wafer, a pattern writing apparatus using electron beams, capable of writing or “drawing” fine circuit patterns, needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of patterns configuring an LSI is in transition from on the order of submicrons to nanometers. One of major factors that decrease the yield of the LSI manufacturing is due to pattern defects on the mask used for exposing and transfer printing an ultrafine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of dimensions of LSI patterns formed on a semiconductor wafer, dimension to be detected as a pattern defect has become extremely small. Therefore, a pattern inspection apparatus for inspecting defects on a transfer mask used in manufacturing LSI needs to be more highly accurate.
As an inspection method, there is known a method of comparing an optical image obtained by imaging a pattern formed on a target object or “sample” such as a lithography mask at a predetermined magnification by using a magnification optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the methods described below are known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into an inspection apparatus, writing data (design pattern data) generated by converting pattern-designed CAD data to a writing apparatus specific format to be input to the writing apparatus when a pattern is written on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. In such inspection methods for use in the inspection apparatus, a target object is placed on the stage so that a light flux may scan the target object as the stage moves in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source through the illumination optical system. Light transmitted through the target object or reflected therefrom forms an image on a sensor through the optical system. The image captured by the sensor is transmitted as measurement data to the comparison circuit. After performing position adjustment of images, the comparison circuit compares measurement data with reference data in accordance with an appropriate algorithm, and determines that there exists a pattern defect if the compared data are not identical.
The pattern inspection apparatus described above acquires an optical image by irradiating a substrate with a laser beam and capturing a transmission image or a reflection image of a pattern formed on the substrate. On the other hand, there has been developed an inspection apparatus which acquires a pattern image by irradiating a substrate with an electron beam to detect a secondary electron emitted from the substrate (for example, refer to Japanese Unexamined Patent Application Publication (JP-A) No. 2011-155119). A pattern inspection apparatus which inspects a pattern formed on the substrate (semiconductor wafer or exposure mask) by using electron beams, the diameter of the electron beam is narrowed to perform scanning on the substrate in order to acquire pattern information. In that case, in order to obtain resolution of a pattern image, generally, the diameter of the electron beam is narrowed down to a defect size to be detected or below the defect size. For example, when inspecting a wafer of 5 nm TN or less, a defect size to be detected needs to be around 1 nm at a minimum. In such a case, the beam diameter of the electron beam is set to be 1 nm or less. Consequently, the size of one pixel of an image becomes 1 nm, and if expressing one pixel in, for example, 256 gray levels (8 bits), one million pixels are needed to acquire an image of a region of 1 μm square, and therefore, data capacity of 1 Mbytes becomes needed. Thus, data capacity of 10000 Tbytes is required to acquire an image of a region of 100 mm square. If trying to complete data transmission or data processing of 10000 Tbyte data capacity within a general inspection time period in order to perform a die-to-die inspection or die-to-database inspection described above, it becomes necessary, according to the present technology, to mount fairly large-scale processors in a box scale. Moreover, even when generating a reference image from design data, it is also necessary to treat the same data capacity described above. Therefore, it is not realistic to manufacture an inspection apparatus of a scale which can process such an amount of data.